Mix Shader Difference?
It should depend on the "factor" of the mix, citing the manual:
"Fac
    The amount of mixing of the bottom socket is selected by the Factor input field (Fac:). A factor of zero does not use the bottom socket, whereas a value of 1.0 makes full use. In Mix mode, 0.5 is an even mix between the two, but in Add mode, 0.5 means that only half of the second socket’s influence will be applied."